Turtle structure anticlines
are a premier trap type in deep water, and delineation of crestal fault
patterns is important in their evaluation. Fault patterns above arched
strata tend to be controlled by the shape of the arch, so folds that look
alike should develop similar fault patterns. Here we compare crestal fault
patterns above two adjacent, very well imaged turtle structures in the
deep-water Lower Congo Basin, Gabon.

The two structures
are similar in profile but have very different geologic histories. The
flanks of Turtle Structure A subsided sequentially, during different stress
regimes. First, the north flank subsided as a rollover into a west-striking
normal fault. Next, the east flank formed as a rollover into a northwest-striking
normal fault. Finally, the south and east flanks subsided by salt withdrawal.
Each episode of flank subsidence formed an independent set of crestal
faults, which cut across one another in map view but which are indistinguishable
in profile.

In contrast, all of
the flanks of Turtle Structure B subsided simultaneously during an episode
of extension and salt withdrawal. Most of the deformation over the crest
of Turtle B is accommodated by a single fault system, which curves to
accommodate subsidence on the two highest relief flanks.

This study illustrates
how dramatically different fault patterns can form above turtle structure
anticlines that are geometrically very similar. Kinematic history may
therefore be much more important than fold geometry in controlling crestal
faulting.